1. Field of the Invention
The present invention generally relates to the conversion of chemical energy to electrical energy, and more particularly, to an electrochemical cell of either a primary or a secondary chemistry. In either case, the cell has a negative electrode of lithium or of an anode material which is capable of intercalating and de-intercalating lithium coupled with a positive electrode of a cathode active material. A nitrate compound is mixed with either the anode material or the cathode active material prior to contact with its current collector. The resulting electrode couple is activated by a nonaqueous electrolyte. The electrolyte flows into and throughout the electrodes, causing the nitrate compound to dissolve in the electrolyte. The nitrate solute is then able to contact the lithium to provide an electrically insulating and ionically conducting passivation layer thereon.
2. Prior Art
In a primary cell, the formation of a surface film on an alkali metal anode, especially when the anode is of lithium, is unavoidable. Therefore, the prior art in U.S. Pat. No. 6,060,184 to Gan et al. teaches providing a nitrate additive in the electrolyte to beneficially modify the anode surface film of an alkali metal primary cell, particularly a lithium cell. The nitrate additive interacts with the lithium anode to form an ionically conductive surface layer of a nitrate salt thereon. This salt is more conductive than lithium oxide which may form on the anode in the absence of the nitrate additive. In fact, it is believed that the lithium nitrate or the lithium salt of the nitrate reduction product on the surface of the anode provides for the existence of charge delocalization due to resonance equilibration at the anode surface. This equilibration allows lithium ions to travel easily from one molecule to the other via a lithium ion exchange mechanism. As a result, beneficial ionic conductance is realized. Similarly, U.S. Pat. No. 6,136,477 to Gan et al. describes the provision of a nitrate additive in the electrolyte of a secondary cell.
However, the present invention is the first known attempt to introduce nitrate additives into the chemistry of the cell by having them leach from the cathode active mixture of the positive electrode for a primary or a secondary cell or from the anode material of a secondary cell. Benefits to this approach are that the nitrate compound in a solid form is easily mixed with the electrode material and, if desired, a conductive diluent and a binder, to form a homogeneous mixture which is easily fabricated into an electrode. A cell is formed when the thusly fabricated negative electrode and positive electrode are activated with an electrolyte. The electrolyte serves to wet the electrode material, causing the nitrate additive to dissolve therein. Then, the electrolyte becomes a vehicle for transport of the nitrate compound from the host electrode to form an ionically conductive surface layer on the lithium in a similar manner as if the nitrate compound had been added directly to the electrolyte according to the prior art. However, in contrast to the prior art Gan et al. patents, the electrode material mixed with the nitrate additive serves to meter its beneficial effects as it gradually leaches from the host electrode.
The present invention relates to both primary and secondary electrochemical cells. An exemplary primary cell is a nonaqueous electrolyte, alkali metal/mixed metal oxide electrochemical cell and, in particular, a lithium/silver vanadium oxide electrochemical cell. Lithium/silver vanadium oxide cells are designed for current pulse discharge applications required in powering an implantable medical device such as a cardiac defibrillator. A defibrillator requires a cell that may run under a light load, device monitoring mode for extended periods of time interrupted by high rate, current pulse discharge during device activation.
Voltage delay is a phenomenon typically exhibited in a lithium/silver vanadium oxide cell that has been depleted of about 40% to about 70% of its capacity and is subjected to current pulse discharge applications. The occurrence of voltage delay is detrimental because it may result in delayed device activation and shortened device life. Rdc build-up is characterized by an increase in cell resistance in lithium/silver vanadium oxide cells that have been depleted of about 50% to about 100% of their capacity. Rdc build-up also results in a lowering of pulse minimum voltages during high rate discharge, which in turn limits the life of the electrochemical cell.
The desirable decrease in both voltage delay and Rdc build-up is realized in primary cells that contain silver vanadium oxide having a nitrate compound mixed therewith. The nitrate compound is mixed with the cathode active material prior to the positive electrode being assembled into the cell. The thusly fabricated positive electrode is electrochemically coupled with a negative electrode and activated with a nonaqueous electrolyte. The electrolyte permeates the positive electrode to wet the cathode active material and serve as a vehicle for dissolving and transporting the nitrate compound to the anode active material. In a primary cell, the nitrate compound reacts with the lithium anode to form an ionically conductive protective film thereon.
In a secondary cell built in a discharged condition, the nitrate compound is mixed with either the cathode active material, preferably of lithium cobalt oxide, or the carbonaceous anode material. The nitrate compound reacts with the lithiated material of the positive electrode and also when the lithium intercalates with the anode material of the negative electrode. The thusly formed nitrate salt at the solid electrolyte interface is responsible for improved cycling efficiency in secondary cells.
These and other objects of the present invention will become increasingly more apparent to those skilled in the art by reference to the following description.